1. Field of the Invention
The present invention relates to an exposure apparatus for exposing a substrate with a pattern image projected by a projection optical system in a state in which at least a part of a space between the projection optical system and the substrate is filled with a liquid. The present invention also relates to a method for producing a device based on the use of the exposure apparatus.
2. Description of the Related Art
Semiconductor devices and liquid crystal display devices are produced by the so-called photolithography technique in which a pattern formed on a mask is transferred onto a photosensitive substrate. The exposure apparatus, which is used in the photolithography step, includes a mask stage for supporting the mask and a substrate stage for supporting the substrate. The pattern on the mask is transferred onto the substrate via a projection optical system while successively moving the mask stage and the substrate stage. In recent years, it is demanded to realize the higher resolution of the projection optical system in order to respond to the further advance of the higher integration of the device pattern. As the exposure wavelength to be used becomes shorter, the resolution of the projection optical system is higher. As the numerical aperture of the projection optical system becomes larger, the resolution of the projection optical system is higher. Therefore, the exposure wavelength, which is used for the exposure apparatus, is shortened year by year, and the numerical aperture of the projection optical system is increased as well. The exposure wavelength, which is dominantly used at present, is 248 nm of the KrF excimer laser. However, the exposure wavelength of 193 nm of the ArF excimer laser, which is shorter than the above, is also practically used in some situations. When the exposure is performed, the depth of focus (DOF) is also important in the same manner as the resolution. The resolution R and the depth of focus δ are represented by the following expressions respectively.R=k1·λ/NA  (1)δ=±k2·λ/NA2  (2)
In the expressions, λ represents the exposure wavelength, NA represents the numerical aperture of the projection optical system, and k1 and k2 represent the process coefficients. According to the expressions (1) and (2), the following fact is appreciated. That is, when the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ is narrowed.
If the depth of focus δ is too narrowed, it is difficult to match the substrate surface with respect to the image plane of the projection optical system. It is feared that the margin is insufficient during the exposure operation. Accordingly, the liquid immersion method has been suggested, which is disclosed, for example, in International Publication No. 99/49504 as a method for substantially shortening the exposure wavelength and widening the depth of focus. In this liquid immersion method, the space between the lower surface of the projection optical system and the substrate surface is filled with a liquid such as water or any organic solvent to utilize the fact that the wavelength of the exposure light beam in the liquid is 1/n as compared with that in the air (n represents the refractive index of the liquid, which is about 1.2 to 1.6 in ordinary cases) so that the resolution is improved and the depth of focus is magnified about n times.
When the exposure process is performed by the liquid immersion method, if any gas portion such as bubbles or the like exists in the liquid (especially on the substrate surface) between the projection optical system and the substrate, it is feared that a pattern image to be formed on the substrate may be deteriorated by the influence of the bubble (gas portion). Such a situation is not limited, for example, to only the case in which the bubble is contained in the liquid to be supplied, but there is also such a possibility that the bubble may be formed in the liquid after the supply. If the image formation failure of the pattern image as described above is left as it is, the failure is consequently found out as a defective product at the stage at which the final device is produced. It is feared that the device productivity may be lowered.
On the other hand, the following situation is also assumed. That is, the gas portion is formed by causing such a state that at least a part of the space between the projection optical system and the substrate is not filled with the liquid due to any cause, for example, any malfunction of a liquid supply unit for supplying the liquid to the space between the projection optical system and the substrate when the exposure process is performed on the basis of the liquid immersion method. That is, it is feared that all of the pattern image or a part thereof may be projected onto the substrate without passing through or not through the liquid. In this situation, there is such a possibility that the pattern image may not be formed on the substrate. If such a situation is left as it is, then it is impossible to found out the defect until the final device is produced, and it is feared that the productivity may be lowered.
In the case of the exposure apparatus based on the use of the liquid immersion method, various measurement operations are sometimes performed via the liquid disposed on the image plane side of the projection optical system. However, if the gas portion exists on the image plane side of the projection optical system, and the space is not filled with a sufficient amount of the liquid, then there is such a possibility that the measurement error appears, and/or it is impossible to perform the measurement in some situations.